This invention relates to a method for producing low density .alpha.-silicon nitride pre-shaped whisker compacts in which the whiskers can be used as the high modulus high-strength reinforcing phase for metal, organic and ceramic matrix phase composites. The invention also relates to a method for producing .alpha.-silicon nitride whiskers for blending with metal, organic and ceramic molding powders to provide a high-strength, high-modulus reinforcing phase for composite parts.
This invention is an improvement that the invention disclosed in U.S. Pat. No. 4,388,255 to Simpson.
The development of fiber reinforced composites has created a demand for reinforcement in the form of elongated, single-crystal fibers known as whiskers. The term whiskers generally applies to any single-crystal fiber having a thickness less than 0.1 mm. and a length to thickness ratio of at least 100.
Whiskers are potentially important in the reinforcement of metals and other materials because of their shape, high-modulus of elasticity and high tensile strength. When dispersed in metals, the elongated shape of whiskers renders them difficult particles to displace to the grain boundaries, thus significantly increasing the creep resistance of the metal. Further, the high elastic modulus and tensile strength of many whiskers makes it possible for them to act as reinforcement for producing materials having superior strength to weight and stiffness to weight properties.
Whiskers can be made from very stiff and light covalent compounds such as carbides, nitrides and oxides, which have elastic moduli higher than most metals and are also often many times stronger than steel, especially in proportion to their weight.
.alpha.-Silicon nitride is known to be a highly desirable reinforcement material which does not react chemically with matrix materials. A number of prior art attempts, however, to produce whiskers of .alpha.-silicon nitride have been extremely costly. Accordingly, the art has sought an improved method for producing .alpha.-silicon nitride whiskers where the resulting product has a high proportion of whiskers to other non-fiberous and less desirable shapes. Furthermore, a method of forming a low density body of silicon nitride that can be infiltrated with a matrix material to form a composite member is also of significant commercial significance.